1. Field of the Invention
This invention relates to apparatus for detecting faults and, more particularly, to electronic detectors for detecting, locating and identifying arc faults and fault energy in a power circuit. The invention also relates to methods for detecting, locating and identifying arc faults and fault energy in a power circuit.
2. Background of the Invention
Traditional protection devices, such as circuit breakers, are described with trip curves (e.g., usually semi-logarithmic representations of current versus time (log)). In the case of a thermo-magnetic circuit breaker, both a thermal element (e.g., responding to relatively lower currents analogous to I2R heating of the power circuit wiring) and an instantaneous magnetic element (e.g., typically responding to a suitable factor above rated current, such as, for example, about 200 A for a 20 A current rating) are combined. Fuses respond analogous to heating of wiring.
Arc faults can occur, for instance, between adjacent bared conductors, between exposed ends of broken conductors, at a faulty connection, and in other situations where conducting elements are in close proximity.
Arc faults in systems can be intermittent as the magnetic repulsion forces generated by the arc current force the conductors apart to extinguish the arc. Mechanical forces then bring the conductors together again so that another arc is struck.
Arc faults in electrical systems are of two types: parallel arcs and series arcs. Parallel arc faults (or sputtering arcs) are line-to-line faults or line-to-ground faults which can occur, for instance, when the insulation on the conductors becomes frayed or is penetrated. Such parallel arc faults can draw considerable instantaneous current that is well above the rated value. However, such arc faults, by virtue of the arc voltage and available current, draw arc current that is below the instantaneous or magnetic trip thresholds of protection in a typical circuit breaker. Also, the intermittent nature of an arc fault can create an average RMS current value which is below the thermal threshold for the circuit breaker.
Series arc faults, on the other hand, occur in a single conductor path, such as, for instance, where a conductor has been cut, or at a loose or poor connection. The current in a series arc depends upon the load and is usually less than the current without the arc. A minimum arc current is typically about 0.5 A.
In parallel arc detection, the integration of activity above a threshold can involve a function of power or current, in order to provide relatively faster response for large amplitude arc currents, while avoiding false trips caused by known loads. For example, a trip signal is generated as a function of accumulated, time attenuated magnitude of step increases in current associated with each striking of the arc current. In this case, arc voltage is basically not considered, thereby preventing calculation of energy. See, for example, U.S. Pat. No. 5,963,405.
U.S. Pat. No. 6,522,509 discloses an arc fault detector suitable for aircraft AC electrical systems or other AC systems operating at higher frequencies, such as 400 Hz. The arc fault detector generates a cumulative sum of amounts by which the AC current in each most recent cyclic interval exceeds the current in the immediately preceding half cycle in absolute magnitude. An arc fault indication is generated when a time attenuated value of this cumulative sum reaches a selected level. This also is a form of signature recognition which, once again, does not quantitatively account for arc energy.
Series arcs in an AC circuit are discriminated from other phenomenon by analyzing the timing between pulses in a second derivative of the current signal. A first timer starts timing upon detection of a first pulse in the second derivative of current signal. Time out of the first timer starts a second timer which times a second interval or window during which a series arc fault will generate a second pulse of opposite polarity to the first pulse. Detection of the first pulse followed by a second pulse of opposite polarity during the window sets a flip-flop to record the event. When a predetermined number of events are counted by a counter within a given time-period set by a third timer, an output signal indicating an arc fault is generated. If the second pulse is generated before the window opens, or a third pulse occurs during the window, the flip-flop cannot be set so that other events such as the switching of a dimmer do not generate a false output signal. See U.S. Pat. No. 5,726,577.
U.S. patent application Ser. No. 10/341,483 discloses the determination of parallel arc location while presuming a known relationship between arc voltage and current. Thus, arc location is determined from measured peak current and the presumed voltage drop from source to arc, knowing the resistivity of the intermediate wiring. Unfortunately, arc voltage is a function of material and gap as well as current. Hence, arc voltage is indeterminate. Also, the peak current is believed to be inconsistent, particularly in low voltage DC systems fed from a battery, due principally to the resistance introduced by the battery itself.
It is known to employ a plurality of distributed sensors to determine the type of arc fault (series or parallel) and to isolate the fault to a zone (between sensors). See, for example, U.S. Pat. No. 5,986,860. However, this requires a plethora of voltage and current sensors and a real time data collection scheme. In this case, the accuracy of any location calculation depends on the number of sensors.
In general, both aerospace and residential applications require a strategy for addressing fault remediation after detection, which becomes problematic due to relatively long conduit runs (e.g., behind walls or fuselages). If the fault results in an open or a short circuit, then the determination of location is straightforward using RF tracing techniques. More often, however, arcing is sporadic and leaves little electrical evidence (as contrasted with physical evidence) of the fault. Arcing may be induced through motion and vibration of marginal wiring and wiring bundles that would, otherwise, be considered “normal”.
In the case of 42 VDC arc faults (e.g., in planned future production automobiles), cost pressures and performance requirements motivate the development of a correlation between arc fault energy and damage to nearby materials and systems. Hence, for maximum effectiveness, trip times should be related to accumulated fault energy just as prior protection devices relate, in some manner, trip curves to material damage/flammability studies.
Accordingly, there is room for improvement in apparatus and methods for determining arc fault energy, location and type.